iPhone 7 makes a hissing sound:
A lot of iPhone 7 owners have been complaining about a mysterious hissing sound coming from the back of the device. It seems to be louder when the iPhone is engaged in heavy lifting, such as when a demanding app or game is running, which suggests that it’s related to the processor. Put your ear next to the Apple logo and run something that requires a lot of processing power to test for yourself. It’s also worth mentioning that many phones and other devices will emit some hissing and other noises when under heavy stress — if you listen closely enough, that is. We just don’t notice it most of the time.
There’s currently no fix for this, but reports suggest that it’s not something that’s present in every iPhone 7, so you might want to return your handset and get a new one. If it’s really bothering you, then contact Apple, your carrier, or your retailer and ask for a replacement.